It all started with traditions. For long time pouring water was a specifically male prerogative, who were supposed to "purify" the women on the first day after Easter Sunday. Nowadays, the tradition is not that gender specific.
There are two types of people in the city: those who anticipate the day more than their birthday and those who dread it as much as going on a bad date. Whichever crowd you belong to, if you are out on that magnificent day, count some splashing in and bring some spare clothes,—the fight is real! Some people pour the water from cars on the go, some take water guns and shoot you in your back, some even make water balloon bombs and splash them through their windows, and everyone, literally everyone accepts this as a fact.
For few recent years, the city has organized a special zone for splashing (the downtown around the Rynok Square), to keep those who ruin the fun with complaints about water far from the happy people. However, that might not save everyone. Maybe that is even for better, who wouldn't like to purify themselves for the upcoming summer season?